Mechanisms and conditions of Ti and Cr incorporation in mantle phlogopite: the results of atomistic simulation

Abstract

Modeling of eight mechanisms for the incorporation of Ti⁴⁺ and Cr³⁺ impurity components into phlogopite was carried out by a semi-empirical method using the GULP (General Utility Lattice Program) software. The calculation of thermodynamic mixing properties in the range of 1–7 GPa and 373–1573 K and the analysis of the structure geometry for the simulated solid solutions provided the following energy-preferred schemes of isomorphic substitution: ^VI(Mg²⁺) + 2^IV(Si⁴⁺) = ^VI(Ti⁴⁺) + 2^IV(Al³⁺) and ^VI(Mg²⁺) + 2^IV(Al³⁺) = ^VI(□) + 2^IV(Ti⁴⁺), ^VI(Mg²⁺) + ^IV(Si⁴⁺) = ^VI(Cr³⁺) + ^IV(Al³⁺), and 3^VI(Mg²⁺) = ^VI(Al³⁺) + ^VI(Cr³⁺) + ^VI(□). It is shown the scheme 2^VI(Mg²⁺) = ^VI(Ti⁴⁺) + ^VI(□) illustrating entrance of Ti with the formation of a vacancy is realized in the case of microconcentrations of Ti only. Accumulation of high-Ti contents associates with the formation of a vacancy in the octahedral site. This provides incorporation of Ti via the schemes ^VI(Mg²⁺) + 2^IV(Al³⁺) = ^VI(□) + 2^IV(Ti⁴⁺) and (Mg, Fe²⁺) + 2OH⁻ = Ti⁴⁺  + 2O²⁻ only. It is shown that incorporation of high-Cr concentrations (> 5.5 wt % Cr₂O₃) is accompanied by an increase in the number of vacancies in the octahedral site with an increase in the proportion of the dioctahedral component K(Al, Cr, □)₂AlSi₃O₁₀(OH)₂.